The invention relates to piezoelectric vibration exciters, e.g. of the kind which may be used to apply ending wave energy to resonant panels to form loudspeakers, and to panel-form loudspeakers employing such exciters. Our International patent application WO97/09842, and counterpart U.S. application Ser. No. 08/707,012, filed Sep. 3, 1996, describe such resonant panel loudspeakers and vibration exciters therefor. The latter application is incorporated herein by reference in its entirety.
Conventional piezoelectric actuators exhibit limited mechanical displacement. The output of conventional piezoelectric devices is limited by the materials"" basically low piezoelectric displacement constant. Thus conventional devices of reasonable thickness (i.e. of the order of a few millimetres) offer only micrometer-sized mechanical output motion.
Many attempts have been made to provide piezoelectric devices having increased mechanical output displacement. xe2x80x9cRainbowxe2x80x9d actuators, xe2x80x9cMoonies,xe2x80x9dunimorphic and bimorphic piezoelectric actuators exhibit greater mechanical output motion. However, even the thinnest ceramic wafers, which exhibit the maximum observed output motion, provide a displacement limited to approximately 1 mm of motion in the z-axis direction for a device that is 3-4 cm long. Additionally, 0.25 mm thick ceramic devices are extremely brittle and fragile so that they are prone to breakage and require special handling. U.S. Pat. Nos. 5,632,841 and 5,802,195 and International application WO96/31333 describe high displacement ferro-electric devices.
It is an object of the invention to provide a piezoelectric device of enhanced output.
According to the invention there is provided a vibration exciter comprising a curved piezoelectric device having an attachment portion at which the device is to be attached to a substrate to be vibrated and at least one free portion remote from the attachment portion, and a mass load provided on at least one of the or each free portion.
The piezoelectric device may be a ferro-electric device and may be pre-stressed. Mass loading such pre-stressed ferroelectric devices may provide a surprisingly large increase in output, of around an order of magnitude, when the devices are used to drive resonant bending wave panels.
The device may be arcuate in shape or may be a generally circular dished device.
From another aspect the invention is a resonant panel-form loudspeaker driven in resonance by a vibration exciter as described above.